As will be developed in greater detail hereinafter, it has already been proposed to combine a silicon-containing compound or raw material with a carbon-containing material, generally a bituminous binder, and to shape the resulting mixture into bodies which can be subjected at high temperatures, especially those of electric furnaces, to a reaction which liberates silicon from the silicon carrier. The term "silicon carriers" as used in this invention denotes SiO.sub.2 carriers, e.g. sand, ground quartzite and the like. The electric furnaces in which the silicon or silicon alloys are produced are generally electric pit furnaces, although other electric furnaces can be used.
With the prior art, a bituminous binder, e.g. coal-tar pitch, is added to the mixture. Briquetting is therefore carried out with the use of a binder. The resulting raw material moldings tend to soften and flow in the electric furnace in which the silicon or silicon alloy is produced.
In the prior art it is also known to carry out briquetting without a binder in the cold state under high pressure. The resulting raw material moldings disintegrate in the electric furnace. In practice, therefore, the use of raw material moldings prepared in the manner described has not been disclosed for the production of silicon or silicon alloys. The following remarks apply in detail to the prior art and the existing problems:
The production of silicon and silicon alloys from SiO.sub.2 used in the form of sand or prepared quartzites is carried out mainly in electric furnaces using an addition of carbon carriers such as coke, petroleum coke, coal, wood charcoal, wood, sawdust and the like. In practice there is a large number of formulations kept very secret by the individual manufacturers. This is a reliable indication that operations are carried out empirically and the preparation of the charge for the electric furnace cannot be carried out in accordance with exact physico-chemical considerations.
Gaseous SiO is formed and reacts with the carbon in the immediate vicinity, although it can escape through gaps in the furnace charge aggregate and condense in the colder parts of the furnace. In any case, both reaction steps require close contact between SiO.sub.2 and reducing agent. The two substances are therefore homogeneously mixed in a finely ground form.
In an electric furnace, however, materials in dust form are difficult to handle so that grinding and homogenization are followed by compaction. This results in the problems already described. Attempts have been made to convert the mixture to appropriate cokes in conventional chamber ovens. This has not been successful, because even if pitches are added, the caking properties of the best high volatile coal are not sufficient to bind more than 20% of the silicon carriers firmly in the coke carbon structure, while stoichiometrically 60 to 70% silicon carriers are required.